The invention relates to an information recording and/or read system comprising a record carrier provided with information tracks which are arranged substantially concentrically relative to a common center, the record carrier being divided into sectors and the tracks of each sector containing address information in the form of optically detectable radially juxtaposed address information patterns, the address information being indicative of the radial position of the track relative to the center, and a recording and/or read apparatus for recording and/or reading information in/from the information tracks, which apparatus comprises a drive means for rotating the record carrier about a center of rotation, means for scanning the information tracks by means of a radiation beam for the purpose of recording and/or reading information in/from the information tracks, a closed-loop tracking system for keeping the radiation beam aimed at the information track to be scanned, and an angular-position detection system for determining the angular position of the record carrier.
The invention further relates to a recording and/or read apparatus and to a record carrier for use in such a system. A conventional system of this type is described in the opening paragraph of European Patent Application 0,096,969 (which corresponds to U.S. Pat. No. 4,674,076). The system described therein utilizes an inscribable record carrier of which each track in each sector is provided with an optically detectable control pattern in the form a "header" arranged at the beginning of the sector. Each control pattern comprises an address information pattern representing the sector address and the track address, and a plurality of optically detectable servo marks situated at predetermined positions within the control pattern for the purpose of tracking. When the control patterns are scanned, a tracking error is derived from the radiation beam reflected during scanning of the servo marks. Depending on the tracking error the tracking system controls the radial position of incidence in such a way that the radiation beam remains at the track to be followed. Such a control system is known as a "sampled servo control-system".
Such a tracking system performs satisfactorily, provided that the tracking errors between two consecutive scans of the servo marks do not become too large. These tracking errors are caused inter alia by an eccentric position of the center of rotation relative to the concentric tracks or by out-of-roundness of the information tracks. Therefore, as a result of an excessive eccentricity or an excessive out-of-roundness, the tracking system may no longer be capable of keeping the scanning beam at the track to be followed. Further, it is to be noted that on account of local record-carrier flaws it may be desirable to disable the tracking system temporarily. In order to enable scanning of the desired track to be continued after passage of the detective portion it is necessary that the radiation beam remains substantially directed at the track to be followed while the tracking system is inoperative.